Exposure to JC Virus (JCV) usually ocurs during childhood and results in a sub-clinical infection, however, in a small number of immunodeficient individuals an infection leads to the fatal demyelinating brain disease progressive multifocal leukoencephalopathy (PML). In addition to its pathogenic potential in humans, JCV has also proven to be a highly oncogenic virus in animals; inoculation of hamsters and primates with JCV results in a wide variety of tumors, some of which are among the more frequent types found in people. The genetic organization of the JCV genome is the same as that of SV40 and BKV and this is reflected in the high degree of homology observed in the 3 viral DNAs. However, these viruses differ in several biological parameters and there are indications that certain regulatory elements may be responsible for these differences. The long term objectives are to investigate 4 questions about the unique properties of JCV: What is the molecular basis for this virus': 1) strict host range in vitro, ii) predilection for brain tissue (lytic growth and transformation), iii) ability to adapt to grow in different cells, iv) ability to induce a wide variety of tumors? The molecular approach to these questions is apparent in the proposal's specific aims: 1. Are certain features of the regulatory region of JCV (e.g. a duplicated TATA box, the absence of the sequence PyPyCCXCCC, a tandem repeat which shares little homology with the SV40 and BKV repeats) responsible for the restricted activity of JCV in vitro? Are JCV's lytic and transforming capabilities enhanced if its regulatory signals are replaced by functional elements from SV40 or BKV? Are the regulatory elements of JCV variants more active than those of prototype JCV? 2. How much do differences in specific regions of the polyomavirus proteins contribute to the diverse biology of these viruses? 3. Does JCV adapt to grow in brain tissue? Is adaptation a factor in the pathogenesis of PML? Does JCV's brain tropism reflect the acquisition of a modified (host?) enhancer sequence that only functions well in brain tissue? Does the strain of JCV that circulates in the population have an enhancer which functions in lung or kidney tissue? The approach to these questions involves recombinant DNA molecules that will be tested for biological activity in cell culture and for the ability to function in a CAT assay to determine the contribution of specific regulatory sequences to JCV biology.